RotateAround without Z changing

i want to rotate camera arount an obj and i need it to do without Z changing.
[syntax=csharp]
void Update(){
Vector3 v3;
v3.x = Input.GetAxes("Vertical");//-_uj.TouchAxisRight().y;
v3.y = Input.GetAxes("Horizontal");//_uj.TouchAxisRight().x;
v3.z = 0;
v3 = _camTransform.TransformDirection(v3);
v3.z = 0;
_camTransform.RotateAround(_target.position, v3, 200* Time.deltaTime);
}[/syntax]
this is the third person view camera and i cannot add something like(Below) at the end of code
[syntax=csharp]
_transform.eulerAngles = new Vector3(_transform.eulerAngles.x, _transform.eulerAngles.y,0);[/syntax]
because it will make rotation and then make fixing, which can be seen by user.
so, how to fix v3 before i put it into RotateAround method

 

Why can't you just set the _camTransform's z to 0 immediately after RotateAround()? Why do you have to set another transform first?

 

ive done it with this

Code (CSharp):

1.  Vector3 v3 = new Vector3(_transform.eulerAngles.x - swipeDelta.y, _transform.eulerAngles.y + swipeDelta.x, 0);
2.         v3.x = ClampAngle(v3.x, minimumY, maximumY);
3.         Quaternion rotationTo = Quaternion.Euler(v3);
4.         _transform.rotation = Quaternion.Slerp(_transform.rotation, rotationTo, Time.deltaTime * _camRotSpeed);